Studies of high-membered two-dimensional Ruddlesden-Popper Cs₇Pb₆I₁₉ perovskite nanosheets via kinetically controlled reactions

Two-dimensional (2D) all-inorganic Ruddlesden-Popper (RP) perovskite Cs7Pb6I19 nanosheets (NSs) were successfully developed for the first time by employing a structural recrystallization process with additional passivation of small organic sulfide molecules. The structure of Cs7Pb6I19 NSs is confirmed by powder X-ray diffraction measurements, atomically-resolved STEM measurements and atomic force microscopy (AFM) studies. Cs7Pb6I19 NSs with a specific n value of 6 exhibits unique absorption and emission spectra with intense excitons at 560 nm due to quantum confinement effects in 2D perovskite slabs. The formation mechanisms of 2D Cs7Pb6I19 NSs and 3D γ-CsPbI3 phases were investigated by in situ photoluminescence (PL) spectroscopy and the activation energies of their formation reactions were calculated to be 151 kJ mol−1 and 95.3 kJ mol−1, respectively. The phase stability of 2D Cs7Pb6I19 NSs can be maintained at temperatures below 14 °C for more than 4 weeks. The overall results indicate that 2D Cs7Pb6I19 NSs demonstrate unique optical properties and structural stability compared with other 3D perovskite materials. We have opened a new path to the future discovery of 2D perovskite structures with metastable phases by using this recrystallization method and the assistance of sulfur-derived organic molecules.